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Advanced detection and methods of gamma and x-ray radiation spectrometry


Advanced detection and methods of gamma and x-ray radiation spectrometry

As we have already read my previous articles, the basic principle of detecting ionizing radiation alpha, beta, gamma consists in ionizing a suitable dilute gas in the tube between the electrodes where the high voltage - the Geiger-Müller tube - is used to measure the basic types of ionizing radiation α, β, γand possibly slightly modified (Cadmium Target) I slow neutrons.

The problem is that it is not able to measure larger radiation flux - its dead time (the regenerative time at which it does not detect other particles) is too high (typically 100uS), which limits the number of particles recorded to approx. 1 million in one second. As a rule, however, the dead time increases with the age of the probe and also with unsuitable (higher) stresses or because of increased radiation (the filler is gradually depleting with the detecting particles). Nitrogen or Argon and borine (Br) , Halogen is used here to rapidly quench the generated discharge during particle detection and carrier gas to increase conductivity. Therefore, it is not suitable for the detection of gamma or retinal radiation, where the intensity usually exceeds the potential of the tube.

Other limitations of the G-M tube are then the impulses themselves, their duration and amplitude are more or less constant, so more advanced methods of impulse processing (spectrometry) can not be used. We will solve this problem later.

For advanced methods of detecting particles, special detectors have been developed, principles are many, but here I will only deal with the scintillation detection method. This is a very common and most or less simple method of detecting ionizing radiation, especially high gamma and x-ray radiation. Pulses have information about particle / photon energy amplitude, so this method is widely used for spectral analysis.

The scintillation method consists of two basic things, the basis of which is a substance in which scintillation occurs - the lightning of the photons in the visible spectrum, these lightings are very weak and thus must be detected and amplified - converted into an electrical impulse. To amplify and detect light flashes from a scintillator, a photomultiplier is used, which uses photoelectric effect and the acceleration of strobed electrons from photocathode converts these flashes into weak electrical pulses. Other methods of light detection, especially semiconductor photo diodes (instead of photomultiplier), can also be used. They are either special photodiodes that contain hundreds of individual diode transitions (SiPM - http://sensl.com/products/) and / or common PN diodes (http://www.hamamatsu.com/us/en/product/application/1505 /4521/4321/S8193/index.html) with a high sensitivity to the spectrum of the scintillator and sufficient speed. However, the signal from these diodes is very weak and must be amplified with precision operational amplifiers, since the diode is connected in a photovoltaic mode, detecting the current it generates. It is also in the order of fA to pA. This very small current is transformed into voltage and then amplified.

The spectral properties of the scintillation detector are determined mainly by the scintillation material itself. It emits the light of a given wavelength (in the range of the photodetector used) and its intensity is proportional to the energy of the incident quantum of radiation. Impaired radiation (especially gamma or x-ray) has, of course, learned energy, which is interpreted by eV = electron-volts. This energy depends on the source of radiation, whether the isotope itself or the voltage (braking radiation) and the target material on the x-ray (characteristic radiation). Alternatively, the type of material, which is typically irradiated with gamma radiation, emitting a characteristic gamma spectrum for that element through backscattered electrons. This method of detection is called retinal fluorescence anaysis (https://en.wikipedia.org/wiki/Xray_fluorescence). On the basis of this spectrum analysis, one can find out what element is - each element has a characteristic radiation energy.

I have already described the basic theory, and since GM-tube detectors have already stopped working, I have decided to build a scintillation detector designed for x-ray fluorescence analysis (I do not think much about the possibility of detecting a strong gamma-ray flow but rather about analyzing the amplitudes Of these pulses). Probably the simplest way is a photomultiplier with a scintillator, the pulses are then amplified appropriately, and spectral analysis takes care of the open-source Theremino MCA program (http://www.theremino.com/en/downloads/radioactivity) which analyzes the pulses through the sound card. Yes, it would definitely be better digital conversion with some A / D converter, but I did not find a suitable program that could work with this data from a port. However, I have this as a subject for a future project.

However, with a photomultiplier and especially a suitable scintillation crystal, it is a problem, it must be optically connected to the photomultiplier and perfectly light-proof against external influences. Buying something like this at home with some piece of crystal is not very effective and the appropriate crystal for photomultiplier is quite expensive. Complete scintillator systems with a photomultiplier cost about 6000-20000 CZK. Moreover, those cheaper are usually old and with limited detection efficiency (often used crystals NaI (TI) are highly hygroscopic and lose detection sensitivity).

That's why I decided to make this detector with a special photodiode that is already wearing a scintillator. This is a Hamatsu S1337-1010BR photodiode including a CsI (TI) scintillator (thallium cesium iodide) which is not as hygroscopic as NaI (TI). Normally, prices are around 10000CZK for such a diode with a scintillator (http://www.hamamatsu.com/us/en/product/application/1505/4521/4321/S8193/index.html) but I managed to get it (though Used) from Ebay by one American dealer for a very affordable price.

The disadvantage of using this photodiode is to detect a very weak current that has to be converted to voltage and amplified. It is fA to pA and so it is necessary to use a precision opamp with FET input. I have found a scheme that is directly created for this purpose. It includes a precise OZ (AD515AL), which I replaced with the new type OPA128LM + AD524. By default, the prices of these trademarks are in thousands of Czech crowns, but they are much cheaper on Ebay.
The signal from the last OZ AD524 is very short, so it has to be slightly modified for the audio card input. This is what the RC Integrator +  amplification.

Then there should be no obstacle to detecting the spectrum using the above-mentioned Theremino MCA. He is directly identified for this detection and displays the spectrum and the appropriate element at the peaks.

Either I can distinguish the individual isotopes - direct detection or, by means of the x-ray-fluorescence analysis, by identifying the element (can not be radioactive). XRF - Retgen-fluorescence analysis builds on the basis of the quantum properties of an atom, when we irradiate a gamma or X-rays element, emitting secondary gamma shots (in the polarization direction of radiation). This method is used for non-invasive determination of material composition, and professional equipment costs hundreds of thousands CZK. For this purpose, my scintillation detector + gamma emitter that irradiates the subject under study is suitable. X-rays and / or gamma-emitting isotopes are used as radiation sources - an Am241 radiator from an ionization smoke detector can also be used.

So far I have a few photos of the production on the others I'm still working on.

UV-C LED diode 280nm/1w

Excitation of CsI(Ti) crystall by UV-C diode

Particle detector CsI(Ti) with PN photodiode and precision OP AMP.